Variable Temperature IR Spectroscopy in the Studies of Oxide Catalysts

Abstract

The paper deals with the development of cell technique for spectral measurements and new possibilities provided by the studies at variable temperatures for obtaining information about the nature and properties of active sites. Difficulties caused by light scattering on the pressed samples of catalysts are discussed. Spectroscopy at low temperatures enables us to broaden the number of probe molecules for acidic or basic sites and to obtain data on the active site concentrations from the band intensities. The necessary values of absorption coefficients can be specified from their correlation with the frequencies of adsorbed molecules. At high surface coverages the measurements are complicated by lateral interactions between the adsorbed molecules. The application of isotopic dilution enables one to distinguish between static interaction, which modifies the catalytic and adsorption properties of surfaces, and dipole coupling. The latter shifts the bands of probe molecules, distorting the surface acidity data. Co-adsorption of acidic and basic molecules leads to the mutual enhancement of adsorption. This is revealed in the phenomena of induced Bronsted acidity and induced basicity or Lewis superacidity. Another important property of surface sites detectable by variable temperature spectroscopy is linkage isomerism of surface species when molecules form with surface sites complexes of unusual structure. Such complexes, e.g. the energetically less favourable O-bonded isomer of CO, could be considered as activated states or intermediates in catalytic reactions. Analysis of the band intensity dependence on temperature and pressure provides valuable information about the thermodynamics of adsorption.